Network Working Group T. Ts'o
Request for Comments: 2953 VA Linux Systems
Category: Informational September 2000
Telnet Encryption: DES 64 bit Output Feedback
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document specifies how to use the data encryption standard (DES)
encryption algorithm in output feedback mode with the telnet
encryption option.
1. Command Names and Codes
Encryption Type
DES_OFB64 2
Suboption Commands
OFB64_IV 1
OFB64_IV_OK 2
OFB64_IV_BAD 3
2. Command Meanings
IAC SB ENCRYPT IS DES_OFB64 OFB64_IV <initial vector> IAC SE
The sender of this command generates a random 8 byte initial
vector, and sends it to the other side of the connection using the
OFB64_IV command. The initial vector is sent in clear text. Only
the side of the connection that is WILL ENCRYPT may send the
OFB64_IV command
IAC SB ENCRYPT REPLY DES_OFB64 OFB64_IV_OK IAC SE
IAC SB ENCRYPT REPLY DES_OFB64 OFB64_IV_BAD IAC SE
Ts'o Informational [Page 1]RFC 2953 DES 64 bit Output Feedback September 2000
The sender of these commands either accepts or rejects the initial
vector received in a OFB64_IV command. Only the side of the
connection that is DO ENCRYPT may send the OFB64_IV_OK and
OFB64_IV_BAD commands. The OFB64_IV_OK command MUST be sent for
backwards compatibility with existing implementations; there really
isn't any reason why a sender would need to send the OFB64_IV_BAD
command except in the case of a protocol violation where the IV
sent was not of the correct length (i.e., 8 bytes).
3. Implementation Rules
Once a OFB64_IV_OK command has been received, the WILL ENCRYPT side
of the connection should do keyid negotiation using the ENC_KEYID
command. Once the keyid negotiation has successfully identified a
common keyid, then START and END commands may be sent by the side of
the connection that is WILL ENCRYPT. Data will be encrypted using
the DES 64 bit Output Feedback algorithm.
If encryption (decryption) is turned off and back on again, and the
same keyid is used when re-starting the encryption (decryption), the
intervening clear text must not change the state of the encryption
(decryption) machine.
If a START command is sent (received) with a different keyid, the
encryption (decryption) machine must be re-initialized immediately
following the end of the START command with the new key and the
initial vector sent (received) in the last OFB64_IV command.
If a new OFB64_IV command is sent (received), and encryption
(decryption) is enabled, the encryption (decryption) machine must be
re-initialized immediately following the end of the OFB64_IV command
with the new initial vector, and the keyid sent (received) in the
last START command.
If encryption (decryption) is not enabled when a OFB64_IV command is
sent (received), the encryption (decryption) machine must be re-
initialized after the next START command, with the keyid sent
(received) in that START command, and the initial vector sent
(received) in this OFB64_IV command.
4. Algorithm
Given that V[i] is the initial 64 bit vector, V[n] is the nth 64 bit
vector, D[n] is the nth chunk of 64 bits of data to encrypt
(decrypt), and O[n] is the nth chunk of 64 bits of encrypted
(decrypted) data, then:
Ts'o Informational [Page 2]RFC 2953 DES 64 bit Output Feedback September 2000
V[0] = DES(V[i], key)
V[n+1] = DES(V[n], key)
O[n] = D[n] <exclusive or> V[n]
5. Integration with the AUTHENTICATION telnet option
As noted in the telnet ENCRYPTION option specifications, a keyid
value of zero indicates the default encryption key, as might be
derived from the telnet AUTHENTICATION option. If the default
encryption key negotiated as a result of the telnet AUTHENTICATION
option contains less than 8 bytes, then the DES_OFB64 option may not
be offered or used as a valid telnet encryption option. If the
encryption key negotiated as a result of the telnet AUTHENTICATION
option is greater than 16 bytes the first 8 bytes of the key should